The present invention relates to a method for making a hollow bicycle crank and the method uses various of molds to quickly manufacture the bicycle crank.
A conventional bicycle crank includes two main types one of which is solid crank and the other is hollow crank. The solid crank is too heavy and needs lots of material so that it is not satisfied by the manufacturers and the customers. The hollow crank is welcomed by the market, however, it takes a long period of time to proceed and includes the steps of forging, rolling, welding and cold forging. The step of rolling especially takes a long time to finish the process. How to develop a proper method to make a hollow crank at a low cost is a concern for the manufacturers.
The present invention relates to a method for making a hollow bicycle crank and comprises a step of forging a piece by putting a piece between a first mold and a second mold. An opening is defined between the first mold and the second mold. A third mold is inserted in the opening by its prong to make a recess in a first end of the piece which becomes a prototype.
A step of forming a hollow first end of the crank by putting the first end of the forged prototype in step into a cone-shaped passage in a fourth mold. A pushing member forces the first end of the prototype to go through the cone-shaped passage from a wide opening to a narrow opening of the cone-shaped passage so as to seal the recess.
A step of thermo-forming by putting the first end of the prototype in a mold under heat and deforming the first end to have an angle relative to a second end of the prototype.
A step of setting the shape of the prototype by putting the first end of the prototype in a cold-forging mold.
The present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a preferred embodiment in accordance with the present invention.